In diagnostic imaging, and more particularly in medical diagnostic imaging of the type using penetrating radiation such as X-rays in combination with a detection system comprising an intensifying screen/photographic film combination, the diagnostic utility of the resulting image is frequently limited by the difficulty of achieving proper visualization of a variety of tissues with a single exposure. For example, the screen/film combination has a limited ability to adequately reproduce the full spectrum of density variations generated by the varying absorption of penetrating radiation by a patient's different tissues.
The chest, for example, contains tissues exhibiting widely varying densities with respect to penetration of radiation, and it is relatively difficult to adequately visualize dense, bony central mediastinal tissues, such as the spine, without overexposing air-filled lung tissues. Even with the wide exposure latitude in today's films, the relatively radiopaque areas of a chest image, such as central mediastinal, subdiaphragmatic and peripheral areas of the lung tend to be underexposed. As a result they are reproduced in the film with insufficient contrast, their exposure levels being primarily within the open "toe" region of the characteristic curve, the "H and D curve", of the film. Increasing the exposure to adequately visualize these denser tissues results in overexposure and loss of detail in the image of the less dense tissues. [For a full discussion of the characteristic curve, or "H and D" curve for short, named after F. Hurter and V. C. Driffield who first developed it in 1890, see The Photographic Theory and Process by C. E. Kenneth Mees, The MacMillan Company, 1942 Edition, pages 201-208. The "toe" region of the H and D curve is that part of the curve referred to in Mees, above, as the "induction period".]
To resolve this system deficiency, one may use a filter to selectively reduce the intensity of the penetrating radiation directed to the patient. This allows more radiation to be directed to relatively more dense tissues. U.S. Pat. No. 4,472,828 shows a typical such approach. In the alternative, the intensifying screen may be manufactured to produce a lower intensity output in areas of the image that will normally be overexposed if a sufficiently strong radiation exposure is given to produce adequate output in the areas that otherwise would fall in the "toe" region of the film H-D curve. One way to accomplish this is to selectively increase the thickness of the phosphor layer on the intensifying screen, thereby increasing the light emitted in the corresponding areas.
Another approach, exemplified by EPO Published Patent Application No. 0 158 787, is to dye the transparent protective layer of the intensifying screen. The dye absorbs light in much the same way as optical filters are used to reduce exposure of traditional photographic films. The dye can be applied by immersing the intensifying screen in the dyestuff solution, and the degree of dying of the protective layer can be varied by immersing the screen to a predetermined depth, and then gradually removing it. In this manner, various simple geometric patterns can be achieved. The color strength can also be made to vary continuously so that there is no boundary line.
A better approach has been disclosed in an article appearing in Investigative Radiology 16, No. 4, 281 (July-August 1981) entitled "Photographic Unsharp Masking in Chest Radiography", by J. A. Sorenson, L. T. Niklason and J. A. Nelson. Dr. Sorenson et al., used two exposures to obtain an X-ray image with adequate diagnostic quality. In a first exposure, an unsharp image of the patient's chest is produced. The unsharp image or "mask" is then placed between the X-ray intensifying screen and a second, unexposed film, and a second exposure given to the patient. The unsharp mask absorbs light from the screen in those areas of the chest that normally are well penetrated, preventing overexposure of those areas and resulting in an improved balance of densities across the entire chest radiograph.
The unsharp image masks the output of the intensifying screen in direct proportion to the original unmasked intensity, thus always shifting the available imaging intensity away from the ends of the H-D curve to the middle, high contrast region. Since the filter image is unsharp, it does not produce artifactual lines or shadows in the final product.
While the method taught by Sorenson et al. may produce the best results, it suffers in that the patient must be exposed to radiation twice to produce a single diagnostic image, and the final radiograph is delayed by the time it takes to prepare and process the unsharp mask. Furthermore, the film bearing the unsharp image must be correctly identified and placed in the cassette in proper alignment. Also, the thickness of the masking film interposed between the screen and the unexposed film acts as a spacer and adversely affects image resolution.